a) Field of the Invention
The present invention relates to a photo isolator suitable for use with optical information processors and the like.
b) Description of the Related Art
A photo isolator using a magnetooptical element is known in the art. FIG. 6 is a schematic diagram showing a conventional photo isolator using a magnetooptical element.
A magnetooptical element 51 is positioned at an optical axis Ox. Polarizers 52a and 52b are disposed on opposite sides of the element 51 with their polarizing axes being deviated by 45 degrees. The magnetooptical element 51 has a function of rotating the polarizing axis of incident light by 45 degrees under the application of a magnetic field M. The rotation direction of linearly polarized light is the same as the direction of current flowing in an electromagnet which generates the magnetic field M.
Light 50 propagating along the optical axis is applied to the input side polarizer 52a and becomes linearly polarized light having a polarizing axis of angle of 0 degree. This linearly polarized light becomes incident to the magnetooptical element 51 whereat the polarizing axis is rotated by 45 degrees. The light outputted from the element 51 becomes linearly polarized light having a polarizing axis with a 45 degree angle. This linearly polarized light transmits through the polarizer 52b because the direction of the polarizing axis of the incident light is the same as the direction of the polarizing axis of the polarizer 52b.
Light propagating in the reverse direction along the optical axis Ox passes through the right side polarizer 52b. If light incident to the polarizer 52b is not being polarized, the polarizer 52b changes the incident light to linearly polarized light having a polarizing axis with a 45 degree angle.
The linearly polarized light having a polarizing axis with a 45 degree angle becomes incident to the magnetooptical element 51 whereat the direction of polarization is rotated by 45 degrees. Namely, the polarizing axis of outputted light is in the direction of angle of 90 degrees. This direction is perpendicular to the polarizing axis of the left side polarizer 52a, so that the light is intercepted by the polarizer 52a.
As described above, the photo isolator shown in FIG. 6 transmits the light propagating to the right side and intercepts the light propagating to the left side. In this manner, a unidirectional light transmission function of the photo isolator is realized.
A photo isolator using a magnetooptical element has an excellent directional property. However, it is necessary to use a magnet for applying a magnetic field to the magnetooptical element, resulting in a large sized apparatus and generating leakage fluxes. In an apparatus using a magnetic field of high intensity, such as a photomagnetic disk, the external magnetic field leaked from the apparatus may affect the magnetic field of the photo isolator, lowering the precision of the photo isolator.